This invention relates to a method of controlling a motor in an electric power assisted steering system.
Electric power assisted steering (EPAS) systems are well known in the prior art. Generally, the steering mechanism of a vehicle couples rotational movement of a steering wheel into movement of the road wheels of the vehicle. An electric motor can be used to assist the driver with the movement of the wheels by applying a torque to the system that is coupled into the steering mechanism. A torque sensor in part of the steering mechanism indicates the torque being input to the steering mechanism by the driver; the system uses this to determine how much assistance torque to apply using the motor.
A standard, prior art, EPAS system can be seen in FIG. 1 of the accompanying drawings. Typically, the driver controls the steering via a handwheel 100. A torque sensor 101 is provided, as discussed above, in the steering mechanism of a vehicle; typically this could be located in the handwheel, steering column or pinion assembly. This produces a torque signal TD indicative of the torque applied to the steering mechanism by the driver. The torque controller 102 uses TD to generate an assistance torque demand TA. This assistance torque TA is indicative of a force to be generated by the motor in order to assist the driver with turning the steering wheel in order to move the road wheels 105.
The assistance torque thus generated is generally scaled so that it represents the reduction that is to be achieved in the torque in the steering column and thus the assistance to the driver. The assistance torque TA is generally dependent upon not only the measured torque TD but also the vehicle speed. Furthermore, the assistance torque TA is generally boosted from the measured torque TD by a non-linear boost function, such as is described in European Patent Application publication no EP 0 947 413.
In the motor controller circuit 103, the assistance torque demand TA is converted into a set of signals for controlling the motor 104 so that it produces an amount of torque proportional to the assistance torque demand TA but scaled by factors depending on the mechanical connection of the motor to the steering mechanism; for example, the mechanical ratio of any gearbox used, the mechanical polarity of the gearbox and the efficiency of the mechanical driveline. In some cases, the steering ratio is a non-linear function of the steering angle; in these instances, it is possible to schedule the calculation according to a measurement of steering angle. In other cases it may be desirable to compensate the conversion between TA and the motor control variables by other parameters that are known to affect the physical parts, for example the motor temperature.
In general, a steering system may comprise a number of transformations between linear (or quasi-linear) motion and rotary motion. Typically in a steering system with a rack & pinion steering gear, the driver will apply a force to the rim of a handwheel that is translated into a torque in the steering column. This torque is substantially transmitted to the pinion of the steering gear (there is some modulation and frictional loss in the intermediate shaft). The rack and pinion mechanism translates the applied pinion torque into a rack force. The rack force is then substantially coupled into the steering arms of the road wheel hubs by a linkage (there can be modulation of the forces by the kinematics of the suspension).
The steering arms translate the linkage forces into a torque that is substantially applied along the steering axis of the suspension and hence onto the contact patch between the tyre and road. An electric power assisted steering (EPAS) system typically measures the input torque applied to the handwheel, column or pinion; and applies assistance power via a mechanism on the column, pinion, steering gear or directly about the road wheel steering axis.
It is possible to relate the various forces in the system to the various torques in the system according to the physical dimensions of the active parts in the mechanism. Similarly the electro-motive force from the assistance motor can be substantially related back to an equivalent torque that is applied on the handwheel. Whereas “force” has been used throughout, we include in that term both linear forces and rotational torques. However, in some embodiments it may be desirable to limit the invention to one or the other. Furthermore, it will be recognised that these quantities can be represented in units that are comparable; therefore this description does not consider the scale factors that are necessary to achieve equivalence.
It is also known, for example from the European Patent Application published as EP 0 640 903, that other controller functions can generate torque demands. These include damping control, active return control and feedback from systems such as lane guidance systems. These generally provide a torque “overlay”; that is, an extra torque offset that is to be added or subtracted from the assistance torque.
Due to the recent proliferation of EPAS systems and electronic control systems in vehicles such as cars in general, there is a growing desire to integrate torque request signals generated from external sources and include them as part of the overall EPAS torque generation process.
In general such external torque requests can be split into 2 cases:                a) those that are introduced with the intention of modifying the handwheel torque that is perceived by the driver, for example, to apply a bias to the driver's steering effort. This will be referred to as “driver overlay”.        b) Those that are introduced with the aim of compensating for disturbances or other physical loads applied to the steering system. For example, the vehicle chassis may transmit unwanted forces into the steering system that can be estimated and directly opposed by the additional torque from the motor. These requests shall be referred to as “output overlay” since the output of the actuator is the important part.        
The nonlinear boost curve in the EPS system causes the assistance torque to be non-linearly related to the driver torque. Hence a driver overlay torque will be non-linearly related to the assistance torque and the output overlay torque. It is usually desirable for any torque overlay strategy to limit the amount of driver torque that is needed to overcome the torque demand from other vehicle control systems.
We can therefore identify at least two problems that may be solved:                i) to modify the assistance torque demanded by the normal controller in a way to give a consistent driver overlay independently of the nonlinear boost curve setting, and        ii) to introduce an output overlay torque that is limited in terms of the size of change in the torque that the driver must apply to the handwheel to overcome it.        